1. Field of the Invention
This invention relates to storage stable polyorganosiloxane compositions. More particularly, this invention relates to one part fluorosilicone compositions curable by a platinum catalyzed hydrosilation reaction. The compositions contain an synergistic combination of 1) an amine type catalyst inhibitor that imparts long term storage stability to one-part curable compositions without interfering with curing of the composition at temperatures of at least 100.degree. C. and 2) an additive that provides long-term resistance to heat-induced hardening of the cured gel. Using this additive at prior art levels results in excessive hardening and discoloration of the gel.
2. Description of Background Art
Polyorganosiloxane compositions that cure by means of a platinum catalyzed hydrosilation reaction to form silicone gels are disclosed, for example, in U.S. Pat. No. 3,020,260, issued to Nelson on Feb. 6, 1962, and in U.S. Pat. No. 4,719,275, issued to Benditt and Maxson on Jan. 12, 1988. The gels described by Nelson are obtained by reacting an organosiloxane containing two silicon-bonded hydrogen atoms per molecule with a vinyl-containing copolymer wherein the repeating units are of the formulae RViSiO, R.sub.2 SiO, and CH.sub.3 R.sub.2 SiO.sub.0.5, R is methyl or phenyl and Vi represents vinyl.
Benditt and Maxson teach preparing a fluorosilicone gel by reacting a polydiorganosiloxane containing fluorinated hydrocarbon radicals bonded to silicon and 2 vinyl or other ethylenically unsaturated radicals per molecule with an organohydrogensiloxane containing at least three silicon-bonded hydrogen atoms per molecule.
It is known that inhibitors for platinum catalyzed hydrosilation reactions can be added to curable fluorosilicone gel compositions such as those of Benditt and Maxson to increase their working time at room temperature, however, these inhibitors also decrease the rate at which these compositions cure at temperatures above about 100.degree. C. While known inhibitors will partially suppress the reactivity of catalyzed compositions containing both vinyl radicals and silicon-bonded hydrogen atoms, these compositions cannot be stored for extended time periods under ambient conditions without either undergoing a substantial increase in viscosity or curing to form a solid gel.
The viscosity of commercially useful one-part curable organosiloxane gel compositions should not increase by more than about 100% during the several weeks that can elapse between the time the composition is prepared and the time at which it is desired to cure the composition. During this period the compositions are stored in locations where the ambient temperature may reach 50.degree. C.
A concurrently filed and copending application naming the same two inventors as the present application is directed to one-part curable fluorosilicone gel compositions of the type described in the aforementioned Benditt and Maxson patent. These one-part compositions meet the foregoing stability requirements for commercially useful materials. The unique feature of these compositions is the presence of a specified group of N,N,N',N'-tetraalkylene diamines as inhibitors for the platinum-containing hydrosilylation catalyst that promotes curing of the compositions.
One of the references discussed in the Background section of the aforementioned copending application is U.S. Pat. No. 3,867,343, which issued to Garden on Feb. 18, 1975. This reference teaches using ammonia, amines and other nitrogen-containing compounds as inhibitors for the platinum catalyzed reaction of silicon-bonded hydrogen atoms with silicon-bonded hydroxyl groups. U.S. Pat. No. 4,281,093, issued to Garden on July 28, 1981, discloses using these inhibitors for the reaction of polyorganosiloxanes containing silicon-bonded hydrogen atoms with polyorganosiloxanes containing silicon-bonded hydroxyl, vinyl or allyl groups. The catalysts for the reaction are organometallic complexes of platinum or rhodium. The longest gel time, i.e., time required for gelling of the reaction mixture at 25.degree. C., reported in U.S. Pat. No. 3,867,343 is 2409 minutes, equivalent to 40.15 hours, and is obtained using alpha-dipyridyl. This value does not take into account the increase in viscosity that typically occurs prior to complete gelation. The viscosity increase could render the composition unsuitable for commercial use as a one-part composition considerably prior to the time gelation occurred.
N,N,N',N'-tetramethylethylenediamine was considerably less effective as an inhibitor than alpha-dipyridyl, imparting a gel time of 1533 minutes (25.5 hours).
One of the shortcomings of the cured fluorosilicone gels described in the aforementioned copending application and the Benditt and Maxson patent is the tendency of the cured gel to undergo additional hardening when exposed to temperatures above about 125.degree. C. for prolonged periods of time.
Copending application Ser. No. 322,865, (now U.S. Pat. No. 4,898,903) filed on Mar. 14, 1989 in the names of Lawrence Fiedler and Diane Rasch and assigned to the same entity as the present application teaches adding up to 0.1 weight percent of copper acetylacetonate to curable fluorosilicone gel compositions of the type disclosed in the aforementioned Benditt and Maxson patent to suppress heat-induced hardening of the cured gel. Applicants attempted to apply the teaching of Fiedler and Rasch to one-part curable fluorosilicone gel compositions containing from 10 to about 100 parts per million (ppm), based on the weight of the curable composition of an N,N,N',N'-tetraalkylene diamine in addition to or in place of the acetylenic alcohols conventionally used to extend the storage life and/or working time of organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction. Applicants discovered that at the concentration range of copper acetylacetonate taught by Fiedler and Rasch the cured gels were unacceptably dark in color and too hard for many intended end use applications, including coatings and encapsulants for fragile electronic devices such as integrated circuits. The gel must remain relative soft to provide the required protection without damaging the delicate leads and other exposed parts of the device. In some applications, particularly in automotive industry, the devices are located in the engine compartment where the temperature can reach 150.degree. C. or above.
One objective of this invention is to increase the resistance to thermally induced hardening of cured fluorosilicone gels prepared from compositions described in the aforementioned copending Maxson and Vanwert application without causing unacceptable discoloration or otherwise adversely affecting the appearance and/or properties of the gel.